Many research laboratories and manufacturers around the world are presently involved in the development of laser warning receivers (LWR) for the purpose of protecting military platforms against laser guided weapons by detecting, identifying and locating the laser sources associated with those weapons. A high angular resolution in the determination of the angle-of-arrival of the laser radiation from laser guided weapons is essential in order to effectively optimize counter measures deployed against those weapons. That type of resolution is, however, only possible with the receivers available at present for powerful laser sources such as laser weapons (LW), laser range finders (LRF) and laser target designators (LTD) which generate relatively high levels of power density on a sensing device. The detection of very low level power (VLPL) sources, such as associated with a laser beam rider (LBR), is still generally characterized by a very low resolution with few of the existing LWR's being capable of detecting radiation from a LBR with an angular resolution better than a quadrant. LBR detection up to now has been usually carried out by dedicated wide field-of-view (FOV) high-gain modules, one per quadrant, which simply detect a LBR source without any high degree of resolution. One problem with VLPL detection is high irradiance level created by the sun in normal daylight conditions as compared to the irradiance of the VLPL source so that the beam from the VLPL source is deeply buried in background radiation.
Various techniques for the detection of laser sources are known that use shadow masks, fibre optic bundles, lenses, video imagery, coherence discriminators or holography to achieve detection. Most of these techniques do not provide a combination of a low detection threshold, high angular resolution, wide FOV and wide waveband coverage as required for effective detection and localization of LBR weapons.
The most straightforward method for detecting laser sources combine a photodetector and a narrow-band filter. The filter reduces the level of background radiation and achieves the low detection threshold required. The bandwidth of the filter is fixed and chosen to match the VLPL source wavelength. A problem associated with this method is the lack of angular resolution which is, typically, limited to a quadrant. These types of methods were, however, successfully combined with a gated video camera to produce a high sensitivity system having a high angular resolution as described by Jacques Dubois in U.S. Pat. No. 5,280,167 which issued on 18 Jan. 1994. The main drawbacks of the system described in U.S. Pat. No. 5,280,167 is the relative complexity and, as a result, the high cost associated with that system.